This invention relates to video encoding and decoding and more particularly to a comb filter for use in encoding and decoding signals.
An NTSC composite video signal is formed by combining a luminance (luma) component and a chrominance (chroma) component. The chroma component is formed by modulating the phase and amplitude of a subcarrier having a frequency of about 3.58 MHz. The frequency of the subcarrier is selected so that if the phase of the subcarrier remains constant from line to line, the subcarrier on a given line is 180.degree. out of phase relative to the subcarrier on the next line. Accordingly, whereas the video signal may be considered as having a horizontal subcarrier frequency of 3.58 MHz, the frequency of the subcarrier in the vertical dimension corresponds to twice the horizontal line duration of the signal, or about 7.87 kHz. For the a
signal, the vertical subcarrier frequency corresponds to four times the horizontal line duration of the signal.
The encoding and decoding processes for video signals employ filters. Referring to FIG. 1, a block diagram of a one-dimensional video encoding/decoding system employed with color television signals in accordance with the prior art, a luma component Y is supplied at input terminal 10 to a delay 12 and then provided to modulator and combiner 14. The modulator and combiner also receives two chroma components I and Q at input terminals 16 and 18 via respective low-pass filters 20 and 22. Delay 12 is provided to compensate for the delay of filters 20 and 22. The modulator and combiner 14 also receives sync, burst and blanking signals 24 and produces an encoded output signal 26.
Once the encoded signal is to be decoded (for example, at a receiver) the encoded signal 26 is supplied to bandpass filter 28 and the output thereof is fed to demodulator and separator 30 thereby producing the output chroma signals I' and Q' at terminals 32 and 34. Encoded signal 26 is also provided to delay 36 (which compensates for the delay of bandpass filter 28) and the output of delay 36 is combined with the output of bandpass filter 28 via subtractor 38 to provide a luma signal which, when passed through delay 40 (to compensate for the delay of demodulator and separator 30), produces the output luma signal Y' at terminal 42. This prior art method based on single dimension notch and bandpass filtering is plagued by cross-color and cross-luminance artifacts.
FIG. 2 is a block diagram of an improved prior art encoding apparatus employing comb filtering. The luma signal Y is supplied via input terminal 10 to a luma comb filter 44 which produces a filtered luma signal, while the chroma components I, Q are fed to input terminals 16 and 18 of a chroma comb filter with modulator 46 which generates a combined filtered chroma signal. The two outputs (filtered luma and combined filtered chroma) are supplied to combiner 48 (also receiving sync, burst and blanking 24) thereby producing encoded signal 50. The luma comb filter 44 has a vertical bandpass filter 52 receiving the luma input and a horizontal bandpass filter 54 receiving the output of vertical bandpass filter 52. The output of horizontal bandpass filter 54 is subtracted from a delayed version of the input luma signal (delayed by delay circuit 56 in order to compensate for delays inherent in filter 52 and filter 54) via subtractor 58 to produce the filtered luma output. The chroma comb filter/modulator 46 includes a horizontal low-pass filter 60 for filtering the I chroma component and a horizontal low-pass filter 62 for filtering the Q chroma component, with the outputs of filters 60 and 62 provided to modulator and combiner 64. The modulated output of the modulator/combiner 64 passes through vertical bandpass filter 66 before being supplied to combiner 48.
FIG. 3 is a block diagram of a corresponding prior art decoding apparatus that may be used in conjunction with encoding in accordance with FIG. 2 wherein the encoded signal 50 is supplied to a chroma comb filter 68 (comprising vertical bandpass filter 70 in series with horizontal bandpass filter 72), which provides a filtered chroma output. The output of the chroma comb filter 68 is supplied to demodulator and separator 74, producing the chroma output components I', Q', at terminals 32 and 34. The chroma comb filter 68 output is also the subtrahend of a subtractor 76 which receives the encoded signal (delayed by delay 78) as the minuend for producing the luma component which, being delayed by delay 80, is supplied at the output 42 as Y'. Delay 78 compensates for delays inherent in chroma comb filter 68. Similarly, delay 80 compensates for delays in demodulator and separator 74.
The comb filtering system in accordance with FIGS. 2 and 3 separates luma and chroma components in two dimensions as illustrated in FIG. 4. The signal has both vertical and horizontal frequency components wherein horizontal frequency components represent the frequency of the signal along a horizontal scan line of the video image and vertical frequency components represent the frequency of samples from one vertical line to corresponding samples from other vertical lines. The horizontal axis of FIG. 4 represents horizontal frequency, with zero frequency at the center of the axis, the horizontal subcarrier appearing at either side of the zero frequency value. Vertical frequency is represented on the vertical axis, with zero frequency at the center of the vertical axis, and the vertical subcarrier frequency appearing at either side of the zero axis. Movement along any direction away from luma center 84 represents a positive increase in frequency. Four quadrants are shown to illustrate filter response to the chroma components I, Q when either positive or negative (+I, +Q, -I, -Q).
Signals having frequencies falling within areas 82 are interpreted as chroma signals (each area 82 is roughly centered about the chroma center frequencies 100) and are passed through to the chroma output of the chroma comb filter 68 (and also suppressed from the luma output). Any signals falling outside of the areas 82 are interpreted as luma signals and suppressed from the chroma output (and included in the luma output). The chroma portions 82 represent the negative three decibel points of the vertical and horizontal bandpass filters.
Several problems arise when employing the simple comb filters in accordance with the prior art. First, in the general areas of reference arrows 86 of FIG. 4, (the areas near the two chroma horizontal subcarrier frequencies and centered vertically in line therewith) vertical high frequencies in chroma would exist, but because of the response of the chroma comb filter 68, chroma does not occupy these areas and the signals are interpreted as luma, which may result in cross-luminance artifacts. Another drawback to the simple comb filter method is the loss of high frequency diagonal luma resolution at reference points 88, near the innermost corners of areas 82, which may give rise to cross-color artifacts.
Other attempts in the prior art have included the use of diagonal shaped filters, but such filters require high complexity and are non-separable. In a separable filter having two-dimensional filtering aspects, the filtering may be performed in one dimension separately from the filtering in the second dimension resulting in less complex filtering operations. For example, an M-tap by N-tap digital separable filter requires M plus N multiplies, but if the filter is non-separable, it would require M times N multiplies, adding considerable cost.